1. Field of the Invention
The present invention relates to groundwater sampling. More particularly, the present invention relates to sampling of groundwater at several vertical positions simultaneously.
2. Discussion of Background
Groundwater monitoring involves the analysis of the constituents present in groundwater and the direction and rate of flow of the groundwater. Since groundwater is a significant source of water for drinking, recreation and irrigation, its supply and constituents are of paramount importance. Contaminants present as a constituent in the groundwater can pose a significant problem, sometimes even in trace amounts. Therefore, groundwater monitoring to detect the presence of contaminants is important in protecting the supply of water.
Groundwater monitoring begins with taking a sample of the water and analyzing it for its constituents. However, to properly characterize a groundwater system, a number of samples must be taken at different locations to ascertain how the concentrations of the constituents vary from one location in the system to another and how they change over time so that the evolution of the system can be traced and predicted.
A groundwater system is three-dimensional, requiring sampling from different locations in a horizontal plane and in the vertical direction. Usually, a series of monitoring wells are dug at preselected locations throughout an area where the groundwater system is of interest. These wells are about four inches (ten centimeters) in diameter. The sides of the wells are shored by the insertion of well casing, which is perforated piping. Groundwater passes through the perforations in the casing into the well.
There are several devices for sampling groundwater at multiple elevations in a single well; typically, these have a vertical series of chambers that each permit entrance of a sample of well fluid. See, for example, the descriptions in U.S. Pat. No. 4,745,801 issued to Luzier, U.S. Pat. No. 4,538,683 issued to Chulick, U.S. Pat. No. 3,254,710 issued to Jensen, and U.S. Pat. No. 2,781,663 issued to Maly et al. Portions of the axial dimension, defined by the axis of the well, are established by sealing the chamber to the well casing using gaskets; these gaskets are inflated to seal against the casing when needed and to disengage the casing when not in use. See Maly, et al. for examples of inflatable and fixed seal. Maly, et al. fill each chamber using a solenoid valve to control admission of groundwater. Then, the array of chambers is removed from the well for analysis rather than pumping the contents of the chambers to a remote location for analysis.
In some cases, the fluid from the chambers is pumped to the surface at the top of the well rather than being removed along with the chambers when they are pulled from the well. In most cases, a pump at the well head is used for pumping groundwater samples to the surface. The use of submersible pumps in samplers, however, is also known. Chulick, cited above, uses a single submersible pump for all levels when depth of the sampler requires it. Chulick samples one level at a time by rotating an inner cylinder until its perforations line up with the level selected; then groundwater passes through the perforations into the sampler.
In an approach different from that of Chulick, Luzier uses small diameter tubes to collect simultaneously samples of groundwater at different depths. Each tube is perforated at a different depth and covered with multiple wraps of stainless steel screen. His pump is located at the surface rather than in each tube.
The inflatable seals used in some multi-level samplers require a source of air or other gas for inflating and also pose the possibility that they may rupture, so their reliability is suspect. Surface pumps rather than submersible pumps can only pump water from a depth of less than 30 feet and therefore limited use to shallower wells.
There remains, however, a need for a simple, flexible apparatus for sampling the groundwater and measuring pressure at different elevations along the axis of the well casing simultaneously and without cross contamination between levels. Preferably, such an apparatus would be field-assembled from modular units to meet for a variety of well configurations and be retrievable following use from one well for use in another. There is also a need for apparatus for determining the rate and direction of contaminant transport in an aquifer which can be done based on measurements of groundwater pore pressure at various locations within the aquifer.